The discovery, development and commercialization of the 2-(2-imidazolin-2-yl)pyridine compounds as herbicidal agents has given new meaning to the term "weed control"; for within this series of compounds it has been found that some are broad-spectrum or total vegetation herbicides with activity in both herbaceous and woody plants while others are highly selective weed control agents useful in the presence of crops.
Several methods for the preparation of herbicidal 2-(2-imidazolin-2-yl)pyridines involve the preparation of pyridine-2,3-dicarboxylic acid anhydrides from pyridine-2,3-dicarboxylic acids. Current methods used to prepare pyridine-2,3-dicarboxylic acids include the hydrogen peroxide-base oxidation of substituted quinolines (U.S. Pat. No. 4,816,588) and the sequential two part oxidation of substituted quinolines under basic conditions (EP-331,899A).
However, it has been found that those methods are not entirely satisfactory for the preparation of pyridine-2,3-dicarboxylic acids. The hydrogen peroxide-base oxidation is extremely exothermic and produces a large amount of foam to handle. In addition, that reaction may "hang-fire"--that is, accumulate a sizeable amount of unreacted reagents which then, suddenly, react all at once, releasing large amounts of heat and heavy foam. Such a reaction could lead to a rapid overpressurization of the vessel and explosive results. Many engineering and administrative safeguards are required to ensure that the reaction is proceeding in a controlled manner and that sufficient cooling and head space for foam are available. These issues present a significant obstacle in designing commercial scale reaction equipment.
A great amount of time and effort has gone into trying to develop a method for the oxidation of substituted quinolines which controls the rate of reaction and, consequently, the heat release and foaming, to what can be handled safely while, at the same time, minimizes reaction time. Conventional design techniques for running this reaction continuously were unsuccessful since the uneven reaction rate causes foaming and large heat releases to occur at irregular intervals causing significant variations in the reaction environment, temperature, and, therefore, flow rates and residence time. Conventional continuous reactor configurations, therefore, gave not only poor control of extent of reaction, but also can produce unsafe "hang-fire" conditions.
It is therefore an object of the present invention to provide a continuous method for the preparation of pyridine-2,3-dicarboxylic acids which minimizes the foam problems associated with the hydrogen peroxide-base oxidations of substituted quinolines.